1. Field of the Invention
The present invention relates to a process for preparing a homogeneous cellulose solution and more particularly, to a process for preparing a homogeneous cellulose solution by forming a mixture where powdered cellulose pulp is mixed and swollen with a liquid N-methylmorpholine N-oxide (hereinafter, referred to as NMMO) hydrate solvent supercooled under its melting point, and by subjecting the mixture to a dissolution treatment, thereby producing a homogeneous cellulose solution having a substantially low decomposition characteristic which is used in a field manufacturing cellulose fibers or films, by an easy and fast method.
2. Description of the Related Art
An NMMO hydrate functions as a cellulose non-solvent (hereinafter, referred to as NMMO non-solvent), when it has a water content of about 18% or more by weight, thereby swelling cellulose, whereas as a cellulose solvent (hereinafter, referred to as NMMO solvent), when it has a water content of about 18% or less by weight, thereby dissolving cellulose.
Conventionally known processes for preparing cellulose solution by using NMMO solvent are as follows:
First, sheets of cellulose pulp are mixed and swollen with NMMO non-solvent having a water content of 22% by weight and then, an excess of water contained in the mixture is distilled and removed, thereby cellulose dissolves (For example, see U.S. Pat. No. 4,142,913, U.S. Pat. No. 4,144,080, U.S. Pat. No. 4,196,282 and U.S. Pat. No. 4,246,221). However, the above-mentioned processes should require distillation of the excess water under a reduced pressure in the presence of cellulose, thereby result in a large amount of energy consumption and a long time preparing solution. As a result, the processes suffer from problems that the decomposition of cellulose and the discoloration of NMMO are serious.
As another example, European Patent No. 356,419 discloses cellulose solution preparation process in which cellulose is swollen in a NMMO non-solvent having a water content of 40% by weight and then, an excess of water is distilled under a reduced pressure in a screw extruder on which a fan type flight is mounted. As yet another example, International Patent Application Publication No. WO 94/06530 discloses cellulose solution preparation process using a thin film evaporator as the pressure-reducing distiller. However, the above-referenced processes suffer from problems that the productivity thereof is low and the process and apparatus are complicated.
Second, sheets of pulp are swollen in a liquid NMMO solvent (having a water content of 15% by weight) at a temperature in the range of 85xc2x0 C. to 95xc2x0 C. over its melting point and the resulting mixture swirls and heats without any concentration process, thereby a cellulose solution was produced. This process is disclosed in U.S. Pat. No. 4,211,574. In this way, a gel film is undesirably formed on the surface of pulp sheet, which serves to inhibit the NMMO solvent from diffusing into the interior of pulp sheet, thereby the production of a homogeneous solution results in failure.
Third, cellulose powder and powdered NMMO solvent are simply mixed and dissolved by means of an extruder, thereby producing cellulose solution (For example, see SU 1645308 A1). In case the two components are mixed in large quantities, however, a part of cellulose powder remains at non-dissolved state in the produced solution, thereby the production of a homogeneous solution results in failure.
On the other hand, a solid NMMO solvent (whose melting point is 78xc2x0 C.) at a room temperature and cellulose pulp pieces are thrown into a disintegration mixing mill and disintegrated to mix at a temperature in the range of 40xc2x0 C. to 100xc2x0 C., thereby pellet type mixing granules are produced. Next, the granules are supplied into an extruder to produce a cellulose solution. For example, this process is disclosed in U.S. Pat. No. 5,584,919. In this case, however, there occur some problems that it is difficult to manage solid NMMO solvent and thus to carry out continuous processes.
Fourth, International Patent Application No. PCT/KR97/00104 discloses a process preparing cellulose solution by a twin-screw extruder. A liquid NMMO solvent (whose melting point is 78xc2x0 C.) at a temperature in the range of 90xc2x0 C. to 100xc2x0 C. is supplied into the first block of the twin-screw extruder, cooled in the second block at a temperature of 75xc2x0 C. Then powdered pulp having an apparent diameter 180 mm is supplied into either the second block or third block at a temperature of 75xc2x0 C., and mixed. A mixture produced is heated to a temperature of 120xc2x0 C., while passing through the sections in order of transfer, agitation, melting, solution homogenization, defoaming and constant discharging. It is, however, in this process, found that the swelling and dissolution of pulp powder occur simultaneously during the transfer and agitation processes. In addition, the block receiving solvent and the block receiving pulp powder are different, so that the twin-screw extruder exhibits low efficiency to arrange at least 9-14 blocks (having a L/D 36-48). Furthermore, since pulp powder has an apparent specific gravity in the range of 0.04 to 0.08, it is conveyed only in small amount, thereby resulting in a low productivity.
As stated above, the conventionally known processes have disadvantages such as low homogeneity of produced cellulose solution, complicated processes and the like. To solve the disadvantages, it is desirable that a liquid NMMO solvent is diffused into the interior of pulp at a fast rate and thus swells the pulp, before cellulose is dissolved in the liquid NMMO solvent. At this time, it is preferable to minimize the dissolution of cellulose by the liquid NMMO solvent and to maximize swelling thereof by the liquid NMMO solvent.
Accordingly, it is an object of the present invention to provide a process for preparing a homogeneous cellulose solution which is capable of producing a cellulose-NMMO mixture where powdered cellulose pulp is first swollen in a liquid NMMO solvent supercooled under its melting point by minimizing the dissolution of cellulose by the liquid NMMO solvent and maximizing the swelling thereof by the liquid NMMO solvent. And thus produced powder of the mixture is subjected to a dissolution treatment, thereby producing a high homogeneous cellulose solution.
The liquid NMMO solvent is produced, for example, in such a method that a large amount of water is evaporated out of a NMMO water solution with a water content of 50% or more by weight under a reduced pressure and the resulting NMMO solution is concentrated. The production principle of the liquid NMMO solvent supercooled under its melting point from the concentrated NMMO solvent as cellulose solvent has close relation to thermal behavior of the NMMO hydrate solvent, that is, the melting behavior, and isothermal and non-isothermal crystallization behavior by cooling of NMMO hydrate solvent.
Table 1 shows thermal behavior of NMMO hydrate produced by subjecting 50% by weight of NMMO water solution made by BASF Co. to pressure reduction, distillation and concentration (An accurate water content in NMMO hydrate was measured by using Karl-Fischer method. Every sample was prepared by sealing a sample in amount of 25 mg to 35 mg in a stainless steel capsule (Perkin-Elmer part 319-0218). Heating and cooling rates were respectively both 5xc2x0 C./min and 10xc2x0 C./min under a nitrogen atmosphere, using DSC 7 (which is a trade name and made by Perkin-Elmer Co.).
According to Table 1, on the basis of NMMO hydrate having a water content of 13.3% by weight (hereinafter, referred to as NMMO monohydrate), if heating and cooling rates are respectively increased from 5xc2x0 C. to 10xc2x0 C. per minute for NMMO hydrate having a water content of 15% to 22% by weight more than NMMO monohydrate, the value of melting point is increased and crystallization temperature is decreased. This generally appears in the measurement of DSC. However, if heating and cooling rates are respectively increased from 5xc2x0 C. to 10xc2x0 C. per minute for NMMO hydrate having a water content of 0% to 12% by weight less than NMMO monohydrate, the value of melting point is increased and crystallization temperature is also increased. When NMMO hydrate solvent is made by the concentration, NMMO monohydrate has a crystal transition to anhydrous NMMO in accordance with the reduction of water content, thereby the transition probably gives affection on the thermal characteristic. Anhydrous NMMO is produced by sublimation of NMMO monohydrate, and at this process, NMMO monohydrate whose melting point is 78xc2x0 C. exhibits the crystal transition to anhydrous NMMO. This appears in NMMO hydrate having a water content less than 13.3% by weight, and specifically, NMMO hydrate having a water content in the range of 13.3% by weight to 11% by weight exhibits slightly different thermal characteristics due to instable crystal transition properties. The characteristics shown in Table 1 are also apparent to FIG. 1. FIG. 1 is a graph illustrating melting and crystallization temperatures versus water content in NMMO hydrate, which is heated and cooled at a rate of 10xc2x0 C. per minute. The dotted line denotes melting point Tm of NMMO hydrate by the heating. The solid line denotes crystallization temperatures Tc1-Tc3 by the cooling. In this case, a few melting points and crystallization temperatures are obtained, dependent upon water content in NMMO hydrate. The symbol xe2x80x98Lxe2x80x99 represents liquid region of NMMO hydrate, and the symbols xe2x80x98C1, C2 and C3xe2x80x99 represent crystalline regions divided from the crystallization temperature appearing when molten NMMO hydrate is cooled. If NMMO hydrate having a water content in the range of 13.3% by weight to 22% by weight is melted and heated to 140xc2x0 C., and then cooled, the crystallization in the NMMO hydrate occurs at the region C3 at a temperature in the range of 0xc2x0 C. to 20xc2x0 C. NMMO monohydrate having the water content of 13.3% by weight has melting point of 80xc2x0 C. at a heating rate of 10xc2x0 C./min, and if it is melted and heated to 140xc2x0 C., and then cooled at a cooling rate of 10xc2x0 C./min, the crystallization of NMMO monohydrate occurs at the region C3 at a temperature of 20xc2x0 C. In case of NMMO hydrate having a water content of 8% by weight, the crystallization of the NMMO hydrate occurs at the region C1 at a temperature of 80xc2x0 C. or more, at the region C2 at a temperature in the range of 20xc2x0 C. to 80xc2x0 C., and at the region C3 at a temperature of 33xc2x0 C. or less. If NMMO hydrate having a water content in the range of 13.3% or less by weight to 3% by weight is heated, a few melting points appear according to the water content from the temperature of 80xc2x0 C., as shown in Table 1. If the NMMO hydrate is heated to a temperature where it is fully melted and then cooled, a few crystallization peaks appear according to the water content thereof. In case of the cooling rate of 10xc2x0 C./min, the first crystallization of the NMMO hydrate occurs at a temperature in the range of 85xc2x0 C. to 143xc2x0 C. (the region C1 in FIG. 1), and if the cooling treatment is continuously carried out, the second crystallization of the liquid NMMO hydrate which has not been crystallized in the region C1 occurs at a temperature in the range of 73xc2x0 C. to 81xc2x0 C. (the region C2 in FIG. 1) and the third crystallization occurs at a temperature in the range of 27xc2x0 C. to 33xc2x0 C. (the region C3 in FIG. 1).
When NMMO hydrate is heated over its melting point and then cooled, it maintains liquid state during a predetermined time until its crystallization occurs. The liquid maintaining time is different, dependent upon the cooling rate of the melted NMMO hydrate. FIG. 2 is a graph illustrating non-isothermal crystallization behavior when NMMO monohydrate having a water content of 13.3% by weight is cooled at a cooling rate in the range of 10xc2x0 C./min to 200xc2x0 C./min at the melted state of 90xc2x0 C., by using DSC.
Table 2 shows crystallization temperatures according to the variation of cooling rate for melted NMMO hydrate solvents having the water contents of 13.3% by weight and 8% by weight.
In case of the cooling rate in the range of 10xc2x0 C./min to 200xc2x0 C./min, monohydrate has crystallization temperatures in the range of 29xc2x0 C. to 20xc2x0 C. That is, in case of the cooling rate of 10xc2x0 C./min, NMMO monohydrate has crystallization temperature difference value of 9xc2x0 C. when compared to that (the crystallization temperature of 20xc2x0 C.) in Table 1. This is because of the difference between thermal history appearing at the time when maximum heating temperatures for obtaining the melted material are 95xc2x0 C. (see Table 2) and 80xc2x0 C. (see Table 1). On the other hand, if NMMO hydrate having a water content of 8% by weight is heated to a temperature of 135xc2x0 C. and then cooled, increase of a cooling rate in the range of 10xc2x0 C./min to 200xc2x0 C./min resulted in decrease of crystallization temperature. Unlike NMMO monohydrate, NMMO hydrate having the water content of 8% by weight exhibits crystallization at temperatures of 30xc2x0 C., 78xc2x0 C. and 99xc2x0 C., respectively, at the cooling rate of 10xc2x0 C./min. As the cooling rate is increased, crystallization temperature becomes gradually low, and at the cooling rate of 60xc2x0 C./min or more, the crystallization occurring at the temperatures of 78xc2x0 C. and 99xc2x0 C. disappears, so that only a single non-isothermal crystallization temperature is observed. This is because crystallization of the NMMO hydrate does not occur at the temperatures of 78xc2x0 C. and 99xc2x0 C. and uniformly occurs at a low temperature of 22xc2x0 C. or less, if the cooling rate is increased to 60xc2x0 C./min or more. Therefore, in case of the NMMO hydrate having a water content of 8% by weight, if it is fully heated to its melting point of 135xc2x0 C. and rapidly cooled at a cooling rate of 60xc2x0 C./min or more, the liquid NMMO solvent supercooled even at a temperature of 22xc2x0 C. or less can be produced.
The liquid maintaining time of NMMO solvent supercooled at a predetermined temperature can be observed from isothermal crystallization behavior where NMMO hydrate is heated over its melting point or more and then rapidly cooled to a predetermined temperature. Table 3 shows liquid maintaining time (crystallization time at a predetermined temperature) of NMMO monohydrate (whose melting point is 78xc2x0 C.) melted and heated to a temperature of 95xc2x0 C. Then NMMO monohydrate is cooled at cooling rates of 20xc2x0 C./min and 200xc2x0 C./min, respectively, from 95xc2x0 C. to predetermined temperatures, which are maintained until crystallization of NMMO monohydrate occurs. In case of the cooling rate of 20xc2x0 C./min, when predetermined supercooling temperatures are 30xc2x0 C., 35xc2x0 C., 37.5xc2x0 C., 38.5xc2x0 C., 40xc2x0 C. and 45xc2x0 C. under its melting point, liquid maintaining time is 2 seconds, 10 seconds, 15 seconds, 27 seconds, 38 seconds and 38 seconds or more, respectively. In case of the cooling rate of 200xc2x0/min, at the same supercooling temperatures as those in the above, liquid maintaining time is 12 seconds, 48 seconds, 55 seconds, 217 seconds, 1800 seconds, and 1800 seconds or more, respectively. Specifically, in case of the cooling rate of 200xc2x0 C./min, if the supercooling temperatures are maintained at 40xc2x0 C., the liquid state is kept even over 1800 seconds.
NMMO monohydrate (water content of 13.3% by weight, and melting point of 78xc2x0 C.)
Crystallization time: the time maintaining liquid phase at a given temperature below 78xc2x0 C. after molten NMMO monohydrate at 95xc2x0 C. is supercooled at a cooling rate.
FIG. 3 is a graph illustrating partly phase change behavior of cellulose on the supercooled liquid region of NMMO hydrate solvent, based upon basic experiments as will be described below. At a temperature of 65xc2x0 C. or more which NMMO hydrate is supercooled and maintained in liquid state, the region xe2x80x98Axe2x80x99 where cellulose is dissolved is formed; at a temperature in the range of 50xc2x0 C. to 65xc2x0 C., the region xe2x80x98A+Bxe2x80x99 where cellulose is dissolved and swollen in a irreversible manner is formed; and at a temperature of 50xc2x0 C. or less, the region xe2x80x98Bxe2x80x99 where cellulose is swollen in a irreversible manner and the region xe2x80x98Cxe2x80x99 where cellulose is reversibly swollen are formed. Since a gel film is formed on a surface of pulp powder in the cellulose dissolution region (the region xe2x80x98Axe2x80x99), it is difficult to produce a homogeneous solution or even if the solution is produced, there are needs for a strong shearing force and time consumption. In the region xe2x80x98A+Bxe2x80x99, where dissolution and irreversible swelling of cellulose co-exist, it is possible to produce the solution in an easier manner than that in the region xe2x80x98Axe2x80x99, but there is a need for a predetermined shearing force, which may result in decomposition of the solution. In the region xe2x80x98Bxe2x80x99, where the irreversible swelling of cellulose occurs, cellulose is not dissolved, but swollen, so that the productivity of pulp powder swollen can be high due to consumption of a minimum shearing force and the shortest time. The resulting cellulose NMMO solution becomes a high homogenous solution where decomposition thereof is reduced at a minimum. In the region xe2x80x98Cxe2x80x99, where the reversible swelling of cellulose occurs, the water content of NMMO hydrate is high and thus, a degree of dissolution of cellulose becomes low, thereby making it difficult to produce cellulose solution having a high concentration. However, if cellulose is swollen and heated even on the hatched portion in the region xe2x80x98Cxe2x80x99 as the reversible swelling region, cellulose solution can be produced.
As can be appreciated from the above, in the thermal behavior of NMMO hydrate solvent having a water content in the range of 8% by weight to 18% by weight that is produced by concentrating 50% by weight of aqueous NMMO solution, if NMMO hydrate solvent is melted and then rapidly cooled to a predetermined temperature, it is kept at a supercooled liquid state during a substantially long time period even at a temperature under its melting point, so that the liquid NMMO hydrate solvent is diffused into the interior of pulp powder and thus serves to swell cellulose, while minimizing dissolution of cellulose.
Based upon the above-discussed contents, therefore, according to the features of the present invention, NMMO hydrate solvent having a water content in the range of 8% by weight to 18% by weight, preferably NMMO hydrate solvent having a water content in the range of 8% by weight to 14% by weight is melted and then supercooled to a low temperature under its melting point and in the region where the dissolution and swelling of cellulose co-exist, preferably in the region where the swelling of cellulose exists, the dissolution of cellulose becomes minimized and the swelling thereof becomes maximized, so that the supercooled liquid NMMO solvent is easily diffused into the interior of pulp powder, thereby producing the swollen mixture and hence, the resulting mixture is heated and dissolved, thereby finally producing a high homogeneous cellulose solution.
To accomplish this and other objects of the present invention, there is provided a process for preparing a homogeneous cellulose solution by using a molten NMMO hydrate solvent, the process comprising the steps of: subjecting the molten NMMO hydrate solvent to a fast cooling process to place the NMMO hydrate solvent in a supercooled state, the supercooled state being under its melting point; and mixing the supercooled liquid NMMO hydrate solvent with powdered cellulose pulp and swelling the resulting mixture.